Prof. Divine Kumah

Department of Physics

North Carolina State University

Host: Gannon

Title: Tuning Transport and Magnetism at Polar Oxide Interfaces

Abstract: Atomic-scale interactions at the interfaces between polar and non-polar transition metal oxides have led to the realization of exciting phenomena including two-dimensional electron gases, superconductivity and interfacial magnetism. However, these interactions may lead to the suppression of electronic and magnetic ordering at interfaces with strong structure-property relationships. By imaging the atomic structure of the interface between polar LaSrMnO3 (LSMO) and non-polar SrTiO3, we identify interfacial structural distortions which are correlated with thickness-dependent metal-insulator and ferromagnetic-paramagnetic transitions in the rare-earth manganites. We show that these structural distortions can be tuned by inserting polarity-matched spacer layers at the LSMO interfaces leading to a stabilization of ferromagnetism in LSMO layers as thin as two unit cells. The stabilized magnetism is found to be independent of strain. We employ a combination of synchrotron X-ray diffraction, temperature-dependent magnetization measurements and X-ray magnetic circular dichroism to elucidate the interplay between structural and spin degrees of freedom in the rare-earth manganites.[1,2] Additionally, we find that by tuning growth and post-growth processing conditions, a two-dimensional electron gas (2DEG) forms at the interface between antiferromagnetic LaCrO3 and SrTiO3 providing a route to design all-oxide heterostructures which couple magnetic ordering with the mobile carriers within the 2DEG.[3] These results demonstrate the intimate role of picometer-scale structural distortions on the physical properties of transition metal oxides and have important implications for designing novel quantum materials.

References

[1] Koohfar et. al., npj Quantum Materials 4 (1), 25 (2019)
[2] Koohfar et. al., Physical Review B 101 (6), 064420 (2020)
[3] Al-Tawhid et. al., AIP Advances 10 (4), 045132 (2020)

Dr. Andrew Mounce

Center For Integrated Nanotechnologies

Sandia National Laboratory

Host: Gannon

Title:  Nanoscale Magnetometry Using Nitrogen Vacancies in Diamond

Abstract: The properties that make nitrogen vacancies in diamond (NVs) good qubits, for example long coherence times at room temperature, also make them excellent sensors for magnetic fields. In this presentation, I will discuss the properties of nitrogen vacancies and how to utilize them for magnetometry of devices and materials with DC magnetic fields and fluctuating fields up to GHz frequencies. To be useful as sensors, NVs must be activated close to the surface of diamond but near the surface nitrogen has a low probability of incorporating with a vacancy to create a magnetically sensitive NV defect. With this in mind, I will present our efforts toward improving NV activation yield to optimize sensitivity.  Finally, I will discuss our recent results as examples of the utility of using nitrogen vacancies for wide-field magnetic imaging for applied physics problems and NV detected nuclear magnetic resonance toward fundamental physics problems.

Prof. Morten Eskildsen

Department of Physics

University of Notre Dame

Host: Gannon

Title: Fission induced vortex lattice disordering in UPt₃

Abstract:  Subjecting a type-II superconductor to a magnetic field will cause the formations of quantized vortices.  Due to their repulsive interaction the vortices will, in an ideal situation, arrange themselves into a perfectly order vortex lattice (VL).  In reality, however, thermal effects and/or pinning to material defects are present, and the balance between these competing factors determine both the structural and dynamic properties of vortex matter.  This leads to a rich phase diagram comprised of both ordered and disordered solid phases and vortex liquids.  While transitions between the different phases are driven by changes of intensive quantities such as the magnetic field or temperature, their locations in the phase diagram are sensitive to the amount of defects in the host superconductor.  This provides an experimental handle with which one can tune the vortex matter phase diagram, e.g by introducing impurities during the material synthesis or by bombardment with heavy ions to create columnar defects post growth.

Here we report on small-angle neutron scattering (SANS) studies of vortices in the topological superconductor UPt₃, and specifically how the VL in this material undergoes a gradual disordering as it is subjected to a beam of cold neutrons.  The disordering occurs on a time scale of tens of minutes, and is attributed to local heating events caused by neutron induced fission of ²³⁵U, which temporarily heat regions of the sample above the critical temperature.  Vortices in the affected regions remain in a disordered configuration after re-cooling, which is most likely a quenched vortex glass.  Moreover, the rate of disordering is proportional to the magnetic field, suggesting a direct relation to collective VL properties such as the elastic moduli.  While the VL does not spontaneously re-order once the local heating has been dissipated it is possible to re-anneal the VL by the application of a damped field oscillation, indicating that no permanent radiation damage of the UPt₃ crystal occur within experimental time scales.  The results demonstrate a novel avenue for vortex matter studies, allowing an introduction of localized and reversible quenched disorder.

Prof. Bruno Uchoa

Department of Physics and Astronomy

University of Oklahoma

Host: Kaul

Title: Unbounded hydrodynamics in nodal-line semimetals

Abstract: The ratio between the shear viscosity and the entropy η/s is considered a universal measure of the strength of interactions in quantum systems. This quantity was conjectured to have a universal lower bound (1/4π)h/kB, which indicates a very strongly correlated quantum fluid. After a general overview on quantum hydrodynamics, which describes the long wavelength deviations of local thermal equilibrium, I will address the quantum kinetic theory for a nodal-line semimetal in the hydrodynamic regime. I will show that the ratio between the shear viscosity and the entropy is unbounded, scaling towards zero with decreasing temperature in the perturbative limit. Due to the large unscreened Fermi surface represented by the nodal-line, the phase space for collisions is greatly enhanced compared to either conventional relativistic systems and metals, resulting in a short hydrodynamic scattering time that is nearly temperature independent (up to logarithmic scaling corrections) and set by the radius of the nodal line. I suggest that the lower bound criteria should be modified to account for unscreened relativistic systems with a Fermi surface.

Professor Herb Fertig

Department of Physics

Indiana University

Host: Kaul

Title:

Exciton Drift from Quantum Geometry

Abstract:

In some situations, excitons – bound particle-hole pairs above an insulating ground state – carry an electric dipole moment, allowing them to be manipulated via coupling to an electric field. Excitons in two-dimensional systems turn out to be fully determined by the quantum geometry of its eigenstates, through a quantity which we call the dipole curvature.  The dipole curvature arises naturally in the semiclassical equations of motion of an exciton in an electric field, yielding a drift velocity akin to that expected for excitons in crossed electric and magnetic fields, even in the absence of a real magnetic field.  In real magnetic fields it yields corrections to the drift velocity expected based on Lorentz invariance.  The effect arises naturally in systems where the environments of the electron and hole are sufficiently different, and is particularly relevant for interlayer excitons in heterostructures -- bilayers of different materials.  We discuss estimates of these effects for simple heterostructure models, including graphene and transition-metal dichalcogenide layers, both with and without magnetic fields.  The last of these turn out to be particularly promising platforms for observing these effects.   We discuss how this quantum geometric drift velocity might be observed in experiment, and possible further consequences that follow from the semiclassical dynamics.

Dr. Igor Zaliznyak

Division of Condensed Matter Physics and Materials Science

Brookhaven National Laboratorty

Host: Gannon

Abstract:

While most solids expand when heated, some materials show the opposite behavior: negative thermal expansion (NTE). NTE is common in polymers and biomolecules, where it stems from the entropic elasticity of an ideal, freely-jointed chain. The origin of NTE in solids had been widely believed to be different, with phonon anharmonicity and specific lattice vibrations that preserve geometry of the coordination polyhedra – rigid unit motions (RUMs) – as leading contenders for explaining NTE. Our neutron scattering study of a simple cubic NTE material, ScF₃, overturns this consensus [1]. We observe that the correlation in the positions of the neighboring fluorine atoms rapidly fades on warming, indicating an uncorrelated thermal motion, which is only constrained by the rigid Sc-F bonds. These experimental findings lead us to a quantitative, quasi-harmonic theory of NTE in terms of entropic elasticity of a Coulomb floppy network crystal, which is applicable to a broad range of open framework solids featuring floppy network architecture [2]. The theory is in remarkable agreement with experimental results in ScF₃, accurately describing NTE, phonon frequencies, entropic compressibility, and structural phase transition governed by entropic stabilization of criticality. We thus find that NTE in a family of insulating ceramics stems from the same simple and intuitive physics of entropic elasticity of an under-constrained floppy network that has long been appreciated in soft matter and polymer science, but broadly missed by the “hard” condensed matter community. Our results reveal the formidable universality of the NTE phenomenon across soft and hard matter [1,2].

[1] D. Wendt, et al., Sci. Adv. 5: eaay2748. (2019).
[2] A. V. Tkachenko, I. A. Zaliznyak. arXiv:1908.11643 (2019).

Professor Pengcheng Dai

Department of Physics and Astronomy

Rice University

Host: Gannon

Title:  Magnetic order and spin excitations in two-dimensional honeycomb lattice van der Waals ferromagnetic CrI3

Abstract:  We use neutron scattering to study magnetic order and spin excitations in honeycomb lattice van der Waals ferromagnet CrI3. We show that ferromagnetic phase transition is first order in nature, and controlled by spin-orbit coupling (SOC) induced magnetic anisotropy, instead of magnetic exchange coupling as in a conventional ferromagnet. We discuss the role of the next-nearest-neighbor Dzyaloshinskii-Moriya interaction and Kitaev’s interaction plays on the spin-wave spectra and the possible presence of topological spin excitations in CrI3.

Professor Erik Henriksen

Department of Physics

Washington University in St. Louis

Host: Kaul

Title:  a-RuCl3 as a 2d crystalline acceptor: modulation doping, pn junctions, and the pursuit of Veselago's lens

Abstract: a-RuCl3 is a layered antiferromagnetic Mott insulator widely thought to host a quantum spin liquid state related to the Kitaev QSL. It can be exfoliated down to monolayer thicknesses and incorporated into van der Waals heterostructures along with graphene and myriad other atomically thin materials. Proximity of a-RuCl3 to graphene leads to a significant charge transfer between the two that, surprisingly, persists even when a thin insulating layer is inserted between them, a phenomenon analogous to modulation doping in epitaxially-grown semiconductors. In addition to constituting a new method of charge control in van der Waals stacks, we are using this to generate near-atomically-sharp pn junctions in graphene that may enable a realization of the electron-optic version of Veselago's lens. 

Professor Alannah Hallas

Department of Physics and Astronomy

Stewart Blusson Quantum Matter Institute

University of British Columbia

Host:  Gannon

Title: Intertwined Dipolar and Quadrupolar Correlations in the Pyrochlore Tb2Ge2O7

Abstract:  Rare earth pyrochlores are model systems for the study of highly frustrated magnetism, in large part, because the majority of these materials have a clean separation of energy scales between their single-ion properties and their collective interactions. Terbium pyrochlores represent the exception, where a low energy crystal field excitation profoundly affects the spin interactions and imbues these materials with especially complex phase behavior. We find that the magnetic states of the terbium pyrochlores evolve over three distinct temperature regimes, strongly suggesting a universality to their low temperature phase behavior. In this talk, I will present a comprehensive heat capacity and neutron scattering study of Tb2Ge2O7 where, for the first time, we have been able to elucidate the nature of each of these three regimes. We find that the complex phase behavior of the terbium pyrochlores originates from not only the usual geometric frustration but also frustration due to the competition between magnetic dipoles and electric quadrupoles.

Professor Hitesh Changlani

Department of Physics 

Florida State University

and The National High Magnetic Field Laboratory

Host:  Kaul

Abstract:

Geometrically frustrated magnets harbor novel phases of strongly interacting quantum matter, including those with fractionalized excitations, such as quantum spin liquids.  While there is tremendous progress on understanding their ground state properties, I will primarily focus on their dynamics by highlighting two directions that my group is pursuing. First, I will present our work (done in collaboration with experimentalists) on a newly synthesized pyrochlore, NaCaNi2F7, which we find to be an almost ideal realization of a spin-1 three-dimensional highly frustrated antiferromagnet with no magnetic order and a continuum of excitations, as seen in inelastic neutron scattering [1]. We determine its effective Hamiltonian and show the presence of characteristic "pinch points”, along with good quantitative agreement at intermediate energy scales, from three different theoretical techniques [2]. In the second part of my talk, I switch my focus to the dynamical non-equilibrium effect of ``quantum scarring” which was first reported in a one dimensional Rydberg atom setup [3], with no known real material analog. I demonstrate that this effect is not restricted to 1D, and can be realized in higher dimensional systems [4], which I explain with the help of an exactly solvable point (that we recently discovered) in the XXZ-Heisenberg model on the frustrated kagome lattice [5]. Within the framework of this proposal, I suggest what would be needed to realize scarring in real materials.

[1] K. W. Plumb, H.J. Changlani, A. Scheie, S. Zhang, J. Krizan, J. A. Rodriguez-Rivera, Y.Qiu, B.Winn, R.J. Cava, C.L. Broholm, Nature Physics, 15, 54-59 (2019)

[2] S. Zhang, H.J. Changlani, K. Plumb, O. Tchernyshyov, R. Moessner, Phys. Rev. Lett. 122, 167203 (2019)

[3] H. Bernien et al., Nature 551, 579–584 (2017); C. Turner et al., Nature Physics 14, 745-749 (2018)

[4] K. Lee, R. Melendrez, A. Pal, H.J. Changlani, Phys. Rev. B 101, 241111(R) (2020)

[5] H.J. Changlani, D. Kochkov, K. Kumar, B. Clark, E. Fradkin, Phys. Rev. Lett. 120, 117202 (2018)